According to a known brake system (see Patent Document 1, for instance), both a normal braking based on a frictional brake action and a regenerative braking using a motor generator can be performed under an electronic control without requiring the intervention of the vehicle operator, and ABS (Antilock Brake System) and VSA (Vehicle Stability Assist) control features are also incorporated in the system. This vehicle brake system employs a motor actuated tandem cylinder, incorporated with a pedal simulator providing a required pedal reaction, which may also be combined with an ABS hydraulic unit and/or a VSA hydraulic unit.
The control unit of the brake system sets a target brake fluid pressure as a reference value of the brake fluid pressure according to a brake pedal stroke, sets a target stroke for the motor actuated cylinder according to a fluid pressure versus cylinder stroke map accounting for the relationship between the fluid pressure and the load (load versus fluid loss property), converts the target stroke into a target motor angle, performs a feedback control on the motor angle so that the motor actuated cylinder may be actuated by the target stroke. Thereby, the fluid pressure commanded by the vehicle operator is produced in the wheel cylinders.
To ensure a favorable tracking behavior of the actual brake fluid pressure to the target brake fluid pressure when the load versus fluid loss property has changed, the control unit of the brake system performs a target brake fluid pressure correction by adding the deviation between the reference brake fluid pressure and the detected actual brake fluid pressure to the target brake fluid pressure to obtain a corrected target brake fluid pressure.
A brake pedal operation amount versus fluid pressure reference map that is used for computing a target brake fluid pressure from the brake pedal operation amount in this proposed system is based on the pedal force versus braking force property of a conventional vehicle where the fluid pressure generated by the master cylinder with a power assist using intake vacuum is directly applied to the wheel cylinders so that the same braking feel or impression as that of the conventional brake system may be achieved. In combination with such a target property, the use is made of a brake pedal operation amount versus pedal force property which is realized by using a pedal simulator, and a brake fluid pressure versus braking force property that can be determined from the vehicle property.